Disentangling the Relative Drivers of Seasonal Evapotranspiration Across a Continental‐Scale Aridity Gradient

Abstract

Evapotranspiration (ET) is a significant ecosystem flux, governing the partitioning of energy at the land surface. Understanding the seasonal pattern and magnitude of ET is critical for anticipating a range of ecosystem impacts, including drought, heat‐wave events, and plant mortality. In this study, we identified the relative controls of seasonal variability in ET, and how these controls vary among ecosystems. We used overlapping AmeriFlux and PhenoCam time series at a daily timestep from 20 sites to explore these linkages (# site‐years >100), and our study area covered a broad climatological aridity gradient in the U.S. and Canada. We focused on disentangling the most important controls of bulk surface conductance (Gs) and evaporative fraction (EF = LE/[H + LE]), where LE and H represent latent and sensible heat fluxes, respectively. Specifically, we investigated how vegetation phenology varied in importance relative to meteorological variables (vapor pressure deficit and antecedent precipitation) as a driver of Gs and EF using path analysis, a framework for quantifying and comparing the causal linkages among multiple response and explanatory variables. Our results revealed that the drivers of Gs and EF seasonality varied significantly between energy‐ and water‐limited ecosystems. Specifically, precipitation had a much higher effect in water‐limited ecosystems, while seasonal patterns in canopy greenness emerged as a stronger control in energy‐limited ecosystems. Given that phenology is expected to shift under future climate, our findings provide key information for understanding and predicting how phenology may impact 21st‐century hydroclimate regimes and the surface‐energy balance.

Net Radiation Drives Evapotranspiration Dynamics in a Bottomland Hardwood Forest in the Southeastern United States: Insights from Multi-modeling Approaches

“…The dominant hardwood vegetation composition at this site was sugar maple (Acer saccharum), basswood (Tilia americana L.), and green ash (Fraxinus pennsylvanica Marsh), mostly different from hardwood community found at RSWMA. However, a much higher effect of precipitation was reported in water-limited ecosystems as BHFs and seasonal cycles of canopy greenness in energy-limited ecosystems in higher latitudes using path analysis [31]. Similarly, in humid boreal regions, VPD and Rn were characterized as major drivers of sap-flow and thus transpiration during growing season as well as during drought [32].…”

Net Radiation Drives Evapotranspiration Dynamics in a Bottomland Hardwood Forest in the Southeastern United States: Insights from Multi-Modeling Approaches

“…The dominant hardwood vegetation composition at this site was sugar maple (Acer saccharum), basswood (Tilia americana L.), and green ash (Fraxinus pennsylvanica Marsh), mostly different from the hardwood community found in the RSWMA. However, a much higher effect of precipitation has been reported in water-limited ecosystems such as BHFs and seasonal cycles of canopy greenness in energy-limited ecosystems in higher latitudes using path analyses [31]. Similarly, in humid boreal regions, VPD and R n were characterized as major drivers of sap flow and thus transpiration during the growing season, as well as during drought [32].…”

Scaling Land‐Atmosphere Interactions: Special or Fundamental?

“…Young et al. (2022) noted how the sensitivity of evapotranspiration across a continent as observed in the Ameriflux network of eddy covariance flux towers (Novick et al., 2018) scales primarily with plant lifecycle phenology in energy‐limited regions, while water limitation dominates elsewhere. Yan et al.…”

“…Longer-term and distributed observations also allow for investigation of scale. Young et al (2022) noted how the sensitivity of evapotranspiration across a continent as observed in the Ameriflux network of eddy covariance flux towers (Novick et al, 2018) scales primarily with plant lifecycle phenology in energy-limited regions, while water limitation dominates elsewhere. Yan et al (2021) look further into spring plant phenology in China across a network of thousands of weather stations and find that large-scale atmospheric teleconnections such as the Atlantic Multidecadal Oscillation imprint themselves on the spring blooming times of individual organisms.…”